1. Field of the Invention
This invention relates to color cathode ray picture tubes (CRTs), and is addressed specifically to an improved means for gaging the distance between the emitting surface of a cathode (K) and the facing surface of an adjacent electrode, commonly known as "G1", in the electron gun of a CRT. The distance between the cathode and G1 electrode is commonly known as the "K-G1 spacing." The function of the G1 electrode is to draw and form the electron beam from the electrons emitted from the cathode, and to control the intensity of the beam.
2. Discussion of the Related Art
A system for gaging and establishing cathode to grid spacing in electron tubes is disclosed in U.S. Pat. No. 2,872,609 to Wheeler. A spacer such as a Mylar shim is inserted between the cathode and the adjacent grid electrode, and is removed after the cathode is fixed in position. Another type of K-G1 gaging system known in the art relies upon the measurement of the capacitance between the cathode and the G1 electrode, with the K- G1 spacing being a function of the amount of capacitance. A system for the direct gaging of K-G1 spacing is disclosed in U.S. Pat. No. 3,848,301 to Gruber. The basis for measurement is the height of the stack of the electrodes comprising the electron gun. The accuracy of the system is vitiated by the inevitable variances in the heights of the stacks of the guns being gaged, a variance that results from cumulative errors in the stacking process.
K-G1 spacing can be measured by an optical system, in which a light beam is projected through the K-G1 area. As described in U.S. Pat. No. 3,667,824 to Tsuneta et al, the light is optically magnified to provide an image of the K-G1 space, with the dimensions of the image being a function of the distance between the electrodes.
Yet another method for establishing K-G1 spacing is by air gaging, as disclosed in U.S. Pat. No. 3,533,147 to Bauer et al. A jet of air is directed into one end of a transparent, calibrated tubular column. An outlet at the opposite end of the column leads to a tubular probe which extends through the aperture of the adjacent grid electrode. Air passing from the probe encounters the surface of the cathode, and the resulting back pressure is measured by means of a float in the calibrated column. The measurement is thus a function of the amount of resistance to the air flow against the cathode surface, as measured in relation to the G1 (or a G2) electrode. The system can measure K-G1 spacing with an accuracy of .+-.0.0002 inch. When the desired K-G1 spacing is attained, the cathode is fixed in place relative the G1 electrode by crimping and welding it to a cathode-supporting structure.
The air gaging system is well suited to gaging K-G1 spacing in guns having oxide-coated cathodes as no physical contact is made with the delicate oxide coating. Air gaging however is not practical in electron guns in which the G1 aperture diameter is smaller than 0.016 inch. The walls of an an air-jet probe capable of passing through a smaller aperture must made so thin that the probe becomes too fragile for practical use under manufacturing conditions. Also, as the probe orifice becomes necessarily smaller, the linearity, and hence the sensitivity of the measurement, is adversely affected.
In a gun having multiple beam channels to form and project multiple beams, the K-G1 spacing must be the same for the K-G1 interface in each beam channel. Otherwise, it will be difficult to "track" the three beams; that is, establish a common electrical K-G1 bias. A common negative bias is required to make the electron beams responsive to the dynamic picture signal operating each beam in conjunction with the common voltages of the electron gun which forms the beams. Otherwise, and by way of example, if the K-G1 spacing differs beyond tolerance among the beams, the picture signal may not sufficiently "cut off" a beam when it is not needed, thus degrading the picture.
Two types of electron-emitting cathodes are in common use in electron guns. One is the oxide-coated cathode which is essentially a metal cup covered with an electron-emitting oxide. A cathode of this type and its manufacture is described in commonly owned U.S. Pat. No. 4,619,168 to Wichman. The oxide-coated cathode is susceptible to damage, as the oxide can flake off under mechanical contact. Also, the number of electrons that can be drawn off from an oxide coating is limited.
The other type of cathode in common use is the dispenser cathode, which essentially comprises a round pellet of molybdenum impregnated with barium oxide. The pellet, which is very hard, is enclosed in a tungsten cup with an aperture pattern in its center for the release of the electrons which form the beam. The dispenser cathode is relatively immune from damage by physical contact.
Both types of cathode are caused to emit electrons by heating them to a high temperature by means of a filament enclosed in the cathode structure. The dispenser cathode can be heated to a higher temperature than the oxide-coated cathode, and hence will emit a greater number of electrons.
Because of its greater electron emission and longer life, the dispenser cathode is particularly favored in high resolution CRTs which require a smaller beam spot on the screen. To achieve this smaller spot size the beam needs a higher current density and hence, a smaller G1 aperture to form a more powerful focusing field for the beam. However, because of the smaller G1 spacing aperture, accurate gaging of the K-G1 spacing is made problematic.